9b Robotic Systems Sensors Part2

7/29/2019 9b Robotic Systems Sensors Part2

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Robotic Systems(9b)

Dr Richard Crowder

School of Electronics and Computer Science


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Remote Centre Compliance - RCC

TheRCCsystem is a passive system, and relies on a springand link structure.

The system can correct for minor error in position, both

rotational and translational.

The design relies on the RCC and the object withstandingthe applied forces


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Remote Centre Compliance

Translation Rotation


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Remote Centre Compliance: Applications

Clearance Fits

Shafts onto bearings

Rivets into holes

Interference Fits

Bushings into housings

Shafts onto laminate stacks


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TheInstrumented RCC

Identical structure as the RCC, with the springs being replace byelastomer, and fitted with sensors.

The resultant movement is used to determine the tips motion throughthe robot, and hence the applied force.

Typical application: tip force is constant, the IRCC will remain in

constant position relative to the surface


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Peg in Hole Problem

Many robotic applications require the insertion of a peginto a hole.

With a fixed system, minor errors in the task model will

lead to a failure to complete.

Fine motion is required to accommodate these errors either by passive compliance or active force control.

Four stages to the process


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Stage 1 - Approach

All forces are zero


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Stage 2 Chamfer Crossing

Constrained motion

Trans(z )by chamfer

Trans(x) by the object

Rot(y) by the object

Z

X

Z


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Stage 3 One-point contact

Rotation around

the Y axis is possible


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Stage 4 Two point contact

Further rotation around the Y axis

To align the hole and peg axis.

At this stage the peg can either

jam or wedge in the hole


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Jamming and Wedging Jamming occurs when the applied forces and moments are incorrect and can be

resolved by the removal of forces

Wedging is geometric and hence the removal of external forceswill notresolve the issue.

For satisfactory insertion:

Initial position error must be less than the width of the chamfer or chamfercrossing will not occur.

To avoid wedging the orientation error must be

where R is the diameter of the hole, r the pegs diameter and thecoefficient of frictionThe forces must be within the jamming parallelogram

R

rR


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Jamming Parallelogram (information only)

Two point contact

One point contact

One point contact

Two point contact

12

1

1

1

z

x

f

f

xrf

M

r2

l


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Force Control

Distance not exactly known

Consider a robot

clearing a window


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Force Control


Robot under position

Control applies enough

force to reach target


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Force Control


Under force control,

Applies a defined force

to the end effector, will

stop if the object can

withstand the force


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Constrains

Natural Constraints

Derived from the compliance frames of the application

Artificial Constraints Derives from the control requirements


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Application of constraintsZ

XY 0v

0

0n0

0f0v

Natural

z

y

zx

yx

z

y

xz

xy

f

0n

0n0f0v

Artificial


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Force Sensor

Normally at least eight sensors

are required to fully resolve

all the forces

[F] = [D][S]

[F] is the force vector

[S} is the sensor vector

[D] is the calibration matrix (6n)


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Maltese-cross Force Sensor

W1 W2W7

W8

W3

W4

W5W6

Fz

Fx

Fy

x

y

z


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8

7

6

5

4

3

2

1

67656361

56

4844

38363432

2521

1713

0000

00000520

000000

0000000000

000000

=

ww

w

w

ww

w

w

cccc

cc

cc

cccccc

cc

FF

F

z

y

x

z

y

x

Solution


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Wrist Sensor


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Hybrid Position-Force Controller

Gain

Control

Control

Arm

ControlJ-1

JT

JT

I-S

I-S

S

S

J

J-1

Torque

ForwardKinematics

Position

Speed

Force

9b Robotic Systems Sensors Part2

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